LATE PLEISTOCENE OSCILLATIONS. 
11 
physical character of the upper clay shows that it was largely 
derived from the erosion of land surfaces by stream and wave 
action. It must have been deposited in a considerable depth of 
water because of its fineness and the even, undisturbed character 
of its bedding. 
The physical character of the Champlain clays in the Ottawa 
district shows that different climatic and physical conditions 
existed during the times of deposition of the lower and upper 
clay. Up to the time of maximum submergence of the 
land the ice-sheet was probably not far distant, but during the 
greater part of the time when emergence of the land was taking 
place the ice had retreated far to the north. The physical 
character of the marine deposits also suggests that the sea rose 
on the land as the ice-sheets retreated. 
The Significance of the Molluscan Fauna of the Champlain Clay . 
Fossil marine shells of species of mollusca occur abundantly 
in the Champlain clays of the Ottawa district, but they are 
confined almost exclusively to the lower clays. The commonest 
and most characteristic fossil shell found in the lower clay is 
Portlandia ( Yoldia ) artica Gray ( - Leda glacialis), from the abun- 
dant occurrence of which in the clay the name ‘'Leda clay” was 
given by J. W. Dawson. The molluscan fauna of the lower 
clays in the Ottawa district also includes Nucula tenuis Mont., 
Macoma calcar ea Chemn., Saxicava rugosa Lin., Natica affinis 
Gmel., Neptunea despecta Lin., Cylichna alba Brown, and 
Astarte compressa Mont. (?). Fossil shells of these species have 
all been found by the writer to occur in the lower clays at various 
localities in the vicinity of Ottawa. They have with one or two 
exceptions been previously reported to occur in the clays of the 
Ottawa district . 1 These species of mollusca are all, in European 
waters, high arctic . 2 They are nearly all, however, known to be 
living in the colder parts of the Gulf of St. Lawrence. A notable 
exception is Portlandia arctica Gray, which is not known to occur 
1 Ami, H. M., Geol. Sum, Can., Ann. Kept., voL XII, 1899, pt. C, Appendix. 
1 Brdgger, W. C., *'0m de Senglaciale og Poat-gladale Niraforondringer 1 Kristianiafeltet 
Norges Gcologiska Unteragelse No; 31, 1901, p. 681. 
